1. Field of the Invention
The present invention relates to a vacuum discharge device, such as a vacuum circuit breaker, a vacuum switch, a vacuum triggertron, a vacuum contactor, a vacuum fuse or a vacuum arrester, and, more particularly, to a vacuum discharge device having an insulating envelop supporting a metallic intermediate shielding tube in a state insulated from two electrodes, and having opposite ends provided respectively with metallic layers having electric potentials respectively corresponding to those of the electrodes.
2. Description of the Prior Art
A vacuum switch as shown in FIG. 1 is disclosed in Japanese Patent Publication No. 59-27050. Referring to FIG. 1, indicated at 1 is a ceramic insulating envelop, namely, a component of a vacuum housing, capable of insulating a path for high voltage while a fixed electrode 6a and a movable electrode 6b are open. The fixed electrode 6a and the movable electrode 6b have electrode rods 5a and 5b, respectively. An annular protrusion 2 is formed on the inner surface of the insulating envelop 1 to hold a metallic intermediate shielding tube 3 and a mounting member 4 so that the intermediate shielding tube 3 is insulated from the electrodes 6a and 6b both when the electrodes 6a and 6b are open and when the same are closed. Metallic layers 7a and 7b are formed by a metallizing process on the opposite ends of the insulating envelop 1, and sealing members 8a and 8b are brazed to the metallic layers 7a and 7b, respectively, to vacuum-seal the insulating envelop 1. The respective potentials of the sealing members 8a and 8b are the same as those of the electrodes 6a and 6b, respectively. A metallic bellows 9 is attached to the central portion of the sealing member 8b. The principal functions of the insulating envelop 1 are (1) serving as a component of the vacuum envelop 1, (2) electrically insulating the electrodes 6a and 6b while the same are separated from each other, and (3) holding the metallic intermediate shielding tube 3 in a state electrically insulated from the electrodes 6a and 6b. The insulating envelop 1 must have abilities (a) to withstand severe heat shocks to which the insulating envelop is exposed in the manufacturing processes, such as a brazing process and an evacuating process, and in cutting off short-circuit current, (b) to inhibit creeping flashover the penetration breakage in a conditioning process which is carried out during evacuation or after evacuation, (c) to maintain the dielectric strength thereof above a predetermined rated dielectric strength even if the material forming the electrodes is deposited over the inner surface thereof due to the repetitive current interrupting operation, (d) to maintain a necessary dielectric strength even if the outer surface thereof is soiled by salt and dust during the use thereof, and (e) to withstand mechanical shocks and vibrations resulting from the closing and opening operation of the electrodes 6a and 6b.
Reduction in size of the vacuum switch has been an increasing demand in recent years. Accordingly, it is an important problem to design an insulating envelop having a minimum size and an optimum construction without sacrificing the requisite functions and performance, when the inside diameter and the diameter of the electrodes are specified.
On the other hand, the conventional ceramic insulating envelop 1 is fabricated generally by the following procedure. Alumina powder is molded in a cylindrical molding by a rubber press forming process, the cylindrical molding is machined in a predetermined shape and size, and then the cylindrical molding having the predetermined shape and size is sintered at about 1650.degree. C. in the atmosphere. A paste containing Mo and Mn as principal components is applied to the opposite ends of the sintered cylindrical molding, the paste applied to the opposite ends of the sintered cylindrical body is dried, and then the paste applied to the opposite ends of the cylindrical molding is baked at a temperature in the range of 1400.degree. to 1500.degree. C. to form the metallic layers 7a and 7b. Then the metallic layers 7a and 7b formed respectively on the opposite ends of the sintered cylindrical molding are plated with Ni to finish the insulating envelop 1.
The sealing members 8a and 8b are brazed at approximately 800.degree. C. respectively to the metallic layers 7a and 7b of the insulating envelop 1. The vacuum switch is assembled from the foregoing parts including the insulating envelop 1, and then the vacuum switch is heated at a temperature higher than 500.degree. C. to evacuate the insulating envelop and the vacuum switch is sealed in a vacuum. In a conditioning process, which is carried out during evacuation or after evacuation, a high voltage is applied across the electrodes to repeat vacuum dielectric breakdown to enhance the dielectric strength. The high voltage applied across the electrodes for vacuum dielectric breakdown is far higher, for example, than 10 kV ac and 70 kV ac respectively for vacuum switches respectively having rated dielectric strengths of 3.3 kV and 3.6 kV.
During the conditioning process, penetration breakage occurs frequently in the insulating envelop 1 of the conventional vacuum switch reducing the yield of the process. Penetration breakage is liable to occur in portions where an electric field is concentrated, namely, portions in the vicinity of the metallic layers 7a and 7b and in the vicinity of the annular protrusion 2.
The vacuum switch is used more than twenty years in a high tension circuit. During such an extended period of use, the outer surface of the insulating envelop 1 is soiled by the ambient atmosphere containing dust and salt, and the inner surface of the insulating envelop is coated with the material forming the electrodes due to frequent current interrupting operation. Therefore, the initial dielectric strength of the insulating envelop 1 is reduced gradually with time and, eventually, the dielectric strength of the insulating envelop reduces below the rated dielectric strength. Consequently, external or internal creeping discharge occurs in the insulating envelop 1 due to the deterioration of the dielectric strength by injuries from salt brought about by typhoons and injuries from moisture brought about by snow or by an abnormal transient voltage applied to the circuit by lightning or in making and breaking the circuit, and thereby penetration breakage is caused in the vicinity of the metallic layers 7a and 7b or in the vicinity of the annular protrusion 2 of the insulating container 1. The penetration breakage is a fatal damage in the vacuum switch.
During the conditioning process the vacuum dielectric breakdown voltage across the electrodes 6a and 6b is increased gradually while vacuum dielectric breakdown occurs between the electrodes 6a and 6b, and the intermediate shielding tube 3. Eventually, external creeping flashover occurs between the sealing members 8a and 8b on the insulating envelop 1. In some cases, the external flashover causes penetration breakage across the wall of the insulating envelop 1 in the end portions 1a and 1b of the insulating envelop 1 or in a portion near the annular protrusion 2. If penetration breakage occurs in the insulating envelop during the conditioning process, the vacuum switch becomes defective and, since it is impossible to repair such a defective vacuum switch, the yield of the manufacturing process is reduced.
Accordingly, the improvement of the yield of the conditioning process for the vacuum switch having the intermediate shielding tube 3 can be achieved by (1) a method of reducing the intensity of the electric field acting on the intermediate shield 3 or (2) a method of preventing the vacuum dielectric breakdown of the intermediate shielding tube 3.
Japanese Utility Model Publication Nos. 58-43152 and 58-43153 disclose vacuum switches as shown in FIGS. 2(a) and 2(b) employing both the foregoing methods (1) and (2). The vacuum switch of FIG. 2(a) is provided with two second intermediate shielding tubes 10a and 10b between a first intermediate shielding tube 3 and two electrodes 6a and 6b. The vacuum switch of FIG. 2(b) is provided with a first intermediate shielding tube 3, two second intermediate shielding tubes 10a and 10b, and two third intermediate shielding tubes 11a and 11b. Stacking the intermediate shielding tubes 3, 10a and 10b, or the intermediate shielding tubes 3, 10a, 10b, 11a and 11b one over another increases the length of the insulating tube 1 of the vacuum switch and makes the construction of the vacuum switch complicated, which deteriorates handling facility of the vacuum switch, increases assembling steps, requires an insulating envelop having an increased inner surface for supporting the intermediate shielding tubes 3, 10a and 10b or the intermediate shielding tubes 3, 10a, 10b, 11a and 11b, and requires complicated heating and evacuating processes. Furthermore, since a voltage must be applied to all the intermediate shielding tubes 3, 10a and 10b or all the intermediate shielding tubes 3, 10a, 10b, 11a and 11b for conditioning, such vacuum switches requires a complicated conditioning process. Accordingly, when such a method or methods of preventing vacuum dielectric breakdown are employed, it is impossible to manufacture a vacuum switch at a reduced manufacturing cost.
Recently, electrode materials having a very high dielectric strength have been developed, which has enabled further miniaturization of the vacuum switch. Therefore, problems with conditioning the electrodes 6a and 6b have already been solved by such electrode materials and vacuum dielectric breakdown between the intermediate shielding tube 3 and the electrodes 6a and 6b has become the principal problem.
Accordingly, development of an insulating tube having a construction which will not allow the penetration breakage of the wall of the insulating tube even if vacuum dielectric breakdown occurs between the intermediate shielding tube 3 and the electrodes 6a and 6b in the conditioning process is strongly desired.
Summarizing the foregoing statement concerning the conventional vacuum switch, the ceramic insulating envelop 1 of the conventional vacuum switch, namely, a vacuum discharge device, is subject to penetration breakage in the wall thereof. In forming the conventional ceramic insulating envelop by forming alumina powder through a rubber press forming process, which is a dry forming process, a pressure is not liable to exerton the annular protrusion 2 having a large wall thickness and thereby pinholes are liable to be formed in the annular protrusion 2, because alumina powder has poor fluidity due to high friction between alumina particles. Accordingly, abnormal concentration of electric field on the pinholes occurs upon the sudden variation of the potentials of the intermediate shielding tube 3 and the holding member 4 due to vacuum dielectric breakdown during the conditioning process, and thereby penetration breakage of the insulating envelop 1 is caused.
Furthermore, since an electric field is inherently liable to be concentrated on the junctions of the sealing members 8a and 8b and the insulating envelop 1 and hence potential varies at a high potential gradient toward the outer and inner surfaces of the insulating envelop 1, the provision of the intermediate shielding tubes 3, 10a, 10b, 11a and 11b as shown in FIG. 2(b) is unable to mitigate satisfactorily the intensity of electric field on the outer surface of the insulating envelop 1. To obviate penetration breakage, electric field mitigating rings, not shown, must be put on the outer surfaces of the opposite ends 1a and 1b of the insulating envelop 1 in subjecting the vacuum switch to the conditioning process, which, however, requires additional work.
Still further, in the conventional vacuum switch, the sealing members 8a and 8b and the insulating envelop 1 are substantially the same in outside diameter and hence partial discharge across the sealing members 8a and 8b is liable to occur along the outer surface of the insulating envelop 1. Therefore, once a needle-shaped partial discharge occurs from either the sealing member 8a or the sealing member 8b, the sealing members 8a and 8b are short-circuited in a moment along a straight line on the outer surface of the insulating envelop resulting in external flashover.
Measures have been taken to obviate the penetration breakage of the insulating envelop in the manufacturing process and in the practical use of the vacuum switch, for example, employment of an insulating envelop having a large creeping length to increase the distance between the sealing members 8a and 8b, or the metallic layers 7a and 7b, employment of an insulating envelop having a large diameter to provide increased gaps respectively between the inner surface of the insulating envelop and the electrodes 6a and 6b and between the inner surface of the insulating envelop and the intermediate shielding tube 3, use of an insulating oil or SF.sub.2 gas as an ambient medium in the conditioning process to increase the external flashover voltage, and covering the sealing members 8a and 8b respectively by electric field mitigating rings for preventing external flashover in the conditioning process.
However, it was found through the close examination of the causes of penetration breakage that the flashover voltage of the inner surface as well as the outer surface must be increased.